Certain robot tasks, such as spot welding, require many short point-to-point motions in which the tool must be completely stopped before the application process (a spot weld in this case) is executed. Cycle time reduction allows the process to be completed quicker, or allows more.welds to be completed in the same amount of time, both important items from the production line perspective.
Since the motion of concern is typically short, the maximum velocity of the axis will not usually be reached. This is termed a short motion. When a short motion is executed, the torque applied to the motor becomes the limiting hardware factor. However, when maximum torque is applied, the jerk induced to the system sometimes causes undesirable vibration.
Since acceleration/deceleration control has direct impact on almost all aspects of motion, for example path accuracy, cycle time, smoothness of motion, to name a few, it is desirable to have a flexible and general acceleration/deceleration control scheme.
U.S. Pat. No. 4,819,184 generally describes a control scheme wherein acceleration is calculated in real-time based on position and velocity, where the parameters are applied within the position feedback loop. This places limitations on the amount of processing that can be done within a reasonable time for the closed loop bandwidth constraints.
More specifically, the '184 patent discloses a position controller for a single axis of a multiarticulated robot which determines the mass moment of inertia of the axes, the coupled mass moment of inertia and the moment caused by gravity. From the relationships between acceleration/deceleration, and the drive motor torque for the different axes, the maximum available acceleration/deceleration for the axis is determined while assuming that the maximum motor torque prevails for each axis.
The prior art also shows the use of second order position profile during acceleration, the position controller being within the position loop of the controller. The second order position profile gives rise to infinite jerk (theoretically), hence it might excite undesirable mechanical resonance, especially for very short motion.
Other prior art calculated maximum acceleration based on dynamics parameters within the servo loop. but there is less advantage to this method.
U.S. Pat. No. 4,769,583 to Goor discloses a method for generating a third-order trajectory in real-time to achieve minimum time path generation. The method uses fixed prescribed bounds for jerk, acceleration and velocity independent of robot and load inertia and gravity. In practice, it is undesirable to use the same fixed bounds regardless of the distance of motion, as this will give rise to higher vibrations especially for very short motions when the robot comes to rest. The profile generated, although third order, is symmetrical. From experimental studies, it is found that an asymmetrical profile provides better performance. Furthermore, this method relies on extensive computational power to carry out its implementation, whereby it uses switching surfaces and switching curves in real-time to determine when to switch to different modes successively to reach its destination position. The U.S. patent to Kurakate et al U.S. Pat. No. 4,908,559, discloses a robot control apparatus which executes a computation with regard to an inertia term included in the motion equation for a robot arm at a predetermined period greater than a drive torque computation period. The U.S. patent to Mizuno U.S. Pat. No. 4,956,594, discloses a robot control method which determines the optimal control conditions based upon the weight of the robot and the weight of the workpiece and the inertia of the robot hand and the workpiece, along with other factors. U.S. Pat. No. 5,049,797 to Phillips, U.S. Pat. No. 5,089,758 to Sogawa and U.S. Pat. No. 5,102,289 to Yokoshima et al disclose apparatus and control strategies, specifically developed for vibration control in a robot arm. The U.S. patents to Tsuchihashi U.S. Pat. No. 4,906,907; Umeda U.S. Pat. No. 5,115,178; Repperger U.S. Pat. No. 5,101,472; Mizuno et al U.S. Pat. No. 5,057,995; Hara U.S. Pat. No. 5,057,756; Froyd U.S. Pat. No. 5,025,385; Nakazumi et al U.S. Pat. No. 4,985,668; and Bleidorn et al U.S. Pat. No. 4,718,078 are of a more general interest.
The prior art has a number of shortcomings. For example, for much of the prior art there is large run-time overhead as the dynamics are calculated within the feedback loop. Also, only second order position profile is used during acceleration; this doesn't give the best performance. Much of the prior art utilizes lookup tables which become very huge when more than one or two axes are involved.
Some prior art provides only symmetric acceleration and deceleration, this prevents adjusting deceleration such that vibration is minimized. Finally, within some prior art, some required information is estimated because the full calculations are too expensive during real-time processing.